The invention relates to a sputtering cathode for coating substrates in cathode sputtering apparatus. Such cathode has a cathode base body and a target disposed thereon composed of nonmagnetic material with a sputtering surface, and a magnet system containing permanent magnets with exposed pole faces for the entry and exit of the magnetic lines of force for the production of a closed tunnel of magnetic lines of force overarching the sputtering surface, and having shielding which does not cover the surfaces serving for sputtering and which overreaches at least the magnet system. In such a cathode,
(a) the target is provided with at least two circumferential projections lying concentrically one within the other, and consisting exclusively of nonmagnetic material, which have the at least one sputtering surface enclosed between them and wall surfaces facing one another whose generatrices are substantially perpendicular to the sputtering surface, and
(b) the pole faces on either side of the projections and of the sputtering surface lying between them are drawn forward in the sputtering direction to such a distance above the plane (E--E) in which the sputtering surface lies, that an important part of the magnetic lines of force issue in a substantially vertical direction from the one face of the one projection and, after crossing the sputtering surface, re-enter the other, opposite face of the other projection in a substantially perpendicular direction.
Sputtering cathodes for product coating are known which have sputtering surfaces in a great number of geometrical shapes. All systems have it in common that a closed tunnel of magnetic lines of force is produced over the sputtering surface by a magnet system situated behind it, the tunnel serving for the concentration of the plasma, and considerably increasing the ionization probability necessary for the formation of a glow discharge. The result in every case is a substantially higher sputtering rate for the same sputtering material--all other process parameters being equal--than without the aid of a magnetic field.
The term, "magnetic tunnel," used herein, is to be understood to mean the tunnel-like enclosure of a spatially defined area by magnetic lines of force, regardless of whether the lines of force follow a curbed course reaching all the way to the sputtering surface, or follow a straight course representing merely a cover over a trough-shaped groove.
The term "projection" is to be understood in relation to the plane "E--E" in which the sputtering surface lies. The direction of the projections consequently is perpendicular to this plane E--E, at least in reference to the surface of each projection that faces the sputtering surface. The expression "projection" furthermore refers to the original (virgin) state of the target, and is not to be confused with the elevations left standing on either side of the erosion pit in the state of the art, for merely due to the angle they form with the plane E--E they do not provide the conditions for a reliable inclusion of the plasma. The projections and the part of the target bearing the sputtering surface form one electrical and mechanical unit, in contrast, for example, to the state of the art according to DE-OS 34 11 536 in which the housing parts projecting on both sides of the target margins are insulated from the target, so that they can adjust themselves to an intermediate potential conditioned by the process parameters. A height amounting to a few dark-space intervals, i.e., about 8 mm, advantageously suffices for the height of the projections.
The statement that an important part of the magnetic lines of force emerge from the one wall surface and re-enter the other in a substantially perpendicular direction is not to be understood to mean strictly a viewing angle of 90 degrees. Experience has shown that a deviation of up to about 15 to 20 degrees from this angle is harmless, so that the angle of emergence and entry can also amount to 70 degrees.
A sputtering cathode of the kind described in the beginning is disclosed in U.S. Pat. No. 4,486,287. In it, however, the permanent magnets are magnetized transversely of the projections, i.e., parallel to the sputtering surface. In the case of a circular sputtering surface, the direction of magnetization is therefore radial. This results on the one hand in a larger outside diameter of the cathode, and on the other hand the permanent magnets and the yokes joining them to the base body occupy a relatively great amount of space in the center of the cathode, so that small cathodes are virtually impossible to make. Similar considerations apply to oblong and rectangular cathodes.
Another disadvantage is that, in the proper use of the target material, the distance between the sputtering surface and the magnetic field at a given moment in time changes, and with it the strength of the magnetic field enclosing the plasma. This can be seen in an undesirable alteration of the characteristic curve of the magnetron, and expresses itself practically in varying rates of sputtering and of deposition on the substrates.
The invention, therefore, is addressed to the problem of devising a sputtering cathode of the kind described in the beginning, which will have a sputtering performance that is more constant in time and will permit the sputtering of thicker targets.